Field of the Invention
This invention relates generally to lithium metal electrochemical cells, and, more specifically, to sulfur-based cathodes in lithium metal batteries with polymeric electrolytes.
Sulfur-based materials are attractive cathode active materials for lithium batteries due to their high lithium capacities. For example the theoretical lithium capacity for elemental sulfur is 1675 mAh/g, and capacities for sulfur compounds can be as high as 800 mAh/g or so, as compared with capacities around 170 mAh/g for conventionally used cathode materials such as lithium iron phosphate. However, lithium batteries that have sulfur-based cathodes tend to have poor cycling stability due to the formation and migration of lithium polysulfide salts (e.g., LiSx, 3<x<8) as well as the formation and/or diffusion of elemental sulfur out of the cathode layer. These unbound sulfur-containing species separate from the cathode layer, causing irreversible capacity loss, and can migrate to the anode and decompose, causing an increase in internal ionic resistance of the cell or outright decomposition of the anode.
Lithium metal-based materials are attractive anode active materials for lithium batteries due to their high specific capacities of 3860 mAh/g. Coupling lithium metal anodes to sulfur-containing cathodes would provide a very high specific capacity cell, and would result in a high specific energy cell. However, stable cycling and safe operation of batteries containing lithium metal have proved elusive, no matter what cathode material is used, due to either a reaction of the lithium metal with the electrolyte or formation of lithium dendrites upon cycling.
Improvements in stability, cyclability and lifetime of lithium-sulfur batteries are usually sought through the use of sulfur composites in which inactive materials are combined with sulfur to prevent diffusion of polysulfide and sulfur species. Examples include using carbon structures or other molecular encaging species that can physically and/or chemically sequester sulfur and/or lithium polysulfides, or can react with sulfur to form immobile species such as graphite or cyclized PAN that chemically sequester the sulfur. Another example is using single-ion conductors that allow transport of Li cations, but not anions or elemental sulfur species. Examples of such single-ion conductors include Li3N, LISICON, LIPON, Thio-LISICON, Li2S—P2S5, and the like. Suppression of lithium dendrites has been attempted by use of high modulus electrolytes such as cross-linked PEO, block copolymer electrolytes, and inorganic conductors.
What is really needed is a way to take full advantage of the high lithium capacity of sulfur-containing cathode materials coupled with lithium metal electrodes to make stable, long life cycle electrochemical cells.